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PowerHA SystemMirror 7.1 and Multicasting Setup Dated: 1st Nov 2011 Table of Contents 1 Introduction ................................................................................................................. 1 1.1 Terminology........................................................................................................ 1 1.2 Reference ............................................................................................................ 2 2 PowerHA SystemMirror 7.1 and Multicasting Setup ................................................. 2 1.3 Multicasting Concepts ........................................................................................ 2 1.3.1 Multicast packet communication .................................................................2 1.3.2 Network Switches ........................................................................................3 1.3.3 Internet Group Management Protocol (IGMP) ............................................3 1.3.4 IGMP Snooping ...........................................................................................4 1.3.5 Multicast routing ..........................................................................................4 1.4 Multicast guidelines for PowerHA SystemMirror 7.1 ........................................ 4 1.4.1 Test Multicasting in Network ......................................................................4 1.4.2 Troubleshooting guidelines ..........................................................................5 1 Introduction PowerHA SystemMirror 7.1 Standard Edition High Availability solution implements clustering using multicast (IP based multicast) based communication between the nodes/hosts in the cluster. Multicast based communication provides for optimized communication method to exchange not only heartbeats, but also allows clustering software to communicate critical events, cluster coordination messages etc in 1 to N method instead of communication 1 to 1 between the hosts. Multicast communication is a well established mode of communication in the world of TCP/IP network communication. However in some cases, the network switches used in the communication path need to be reviewed and enabled for multicast traffic to flow between the cluster hosts through them. This document explains some of the network setup aspects that may need to be reviewed before the PowerHA SystemMirror 7.1 cluster is deployed. 1.1 Terminology Multicast Address range Multicast routing (mrouted) L2 Multicast Address 224.0.0.0 through 239.255.255.255 Multicast router protocol. This router handles the multicast packets and routes as necessary. Some switches are capable of routing multicast packets between switches in the cascaded environments (See reference #2) IPv4 Multicast packets map to a predefined IEEE MAC Address that has the format: 01.00.5e.xx.xx.xx Page 1 of 6 PowerHA SystemMirror 7.1 and Multicasting Setup 1.2 Dated: 1st Nov 2011 Reference 1. IP multicasting Information: http://www.ietf.org/rfc/rfc2365.txt?number=2365 2. Multicast address range & Assignments: http://www.iana.org/assignments/multicast-addresses/multicastaddresses.txt 3. Multicast Router Information: http://www.rfc-editor.org/rfc/rfc1075.txt 4. Multicast: http://www.rfc-editor.org/rfc/rfc1054.txt 5. Switch Guidelines(Vendor) a. Information on multicasting setup: http://www.cisco.com/en/US/products/hw/switches/ps708/products_tec h_note09186a008059a9df.shtml b. Switch Support information http://www.cisco.com/en/US/tech/tk828/technologies_tech_note09186a 0080122a70.shtml#NetPro c. 2 PowerHA SystemMirror 7.1 and Multicasting PowerHA SystemMirror 7.1 uses a new re-architected cluster health management layer embedded as part of the Operating System called Cluster Aware AIX (CAA). CAA uses kernel level code to exchange heartbeats over network, SAN fabric (when right Fiber Channel adapters are deployed) and as well as a disk based messaging through the Central repository. Cluster Aware AIX exploits multicast IP based network communication mechanism to communicate between the various nodes in the cluster within a site. Administrator can manually configure a multicast address to be used for the cluster communication or allow PowerHA SystemMirror/CAA to choose a multicast address. Note that multicast communication is used during the initial discovery phase when the cluster is being created, but also during the normal operations of the cluster. Hence it is extremely important that the multicast traffic to flow between the cluster hosts in the datacenter before the cluster formation can be attempted. Please plan to test and verify the multicast traffic flow between the would-be cluster nodes before even attempting to create the cluster. Review the guidelines in the following sections to test the multicast packet flow between the hosts. 1.3 Multicasting Concepts 1.3.1 Multicast packet communication Multicasting is a form of addressing, where a group of hosts form a group and exchange messages. A multicast message sent by one in the group is received by all in the group. This allows for efficient cluster communication where many times messages need to be sent to all the nodes in the cluster. For example a cluster member may need to notify the rest of the nodes about a critical event and can accomplish the same by sending a single multicast packet with the relevant information. Page 2 of 6 PowerHA SystemMirror 7.1 and Multicasting Setup 1.3.2 Dated: 1st Nov 2011 Network Switches Hosts communicate over the network fabric that could consist of many switches and routers. Switch connects different hosts and network segments and allows for network traffic to get sent to the right place. Switch refers to a multi-port network bridge that processes and routes data at the data link layer (layer 2) of the OSI model. Some Switches can also process data at the network layer (Layer 3). Typically a datacenter networking environment consists of hosts connected through network fabric that consists of Ethernet or such cabling and switches. Many times switches are interconnected to form the fabric between the hosts. When switches cascade, multi cast packet would have to flow from Host in the cluster to a switch and then through the other switches to finally reach the destination Host in the cluster. Since switches review multicast packets differently as compared regular network communication, switch to switch communication might not happen for multicast packets if the setup is incorrect. Cluster Host A Switch 1 Host B Switch 2 Figure 1: Hosts connected through cascaded switches Fig 1 shows one such configuration where Host A that would be part of the cluster is connected to Host B through two switches (Switch 1 and Switch 2). Network link between the two switches shown in Red in the picture should allow for the multicast traffic to flow for the cluster to be formed and operational. 1.3.3 Internet Group Management Protocol (IGMP) IGMP is a communications protocol that enables Hosts (receivers) to inform a multicast router (IGMP querier) of the Host’s intention to receive particular multicast traffic. So this is a protocol that runs between a router and Hosts and allows: Page 3 of 6 PowerHA SystemMirror 7.1 and Multicasting Setup Dated: 1st Nov 2011 Routers to ask Hosts if they need a particular multicast stream (IGMP query) Hosts to tell or respond to the router if they seek a particular multicast stream (IGMP reports) So IGMP communication protocol is used by the Hosts and the adjacent routers on IP networks to interact and establish ground rules for multicast communication in particular establish multicast group membership. Switches featuring IGMP snooping derive useful information by observing these IGMP transactions between the Hosts and Routers. This enables the switches to correctly forward the multicast packets when needed to the next switch in the network path. 1.3.4 IGMP Snooping IGMP snooping is an activity performed by the switches to track the IGMP communications related packet exchanges and adapt the same in regards to filtering the multicast packets. Switches monitor the IGMP traffic and allow out the multicast packets only when necessary. Switch typically builds an IGMP snooping table that has a list of all the ports that have requested a particular multicast group and uses this table to allow or disallow the multicast packets to flow. Switches allow many cases to be configured around this IGMP snooping 1.3.5 Multicast routing The network entities that forward multicast packets using special routing algorithms are referred to as mrouters. Also, router vendors may implement multicast routing, so the customer should also refer to the router vendor's documentation and guidance. Hosts and other network elements implement mrouters and allow for the multicast network traffic to flow appropriately. Note that some traditional routers would also support multicasting packet routing. When switches are cascaded(chained), it may be necessary to setup the switch to forward the packets as necessary to implement the mrouting. However this could be one of the possible approaches to solving multicast traffic flow issues in the environment. Refer to the Switch Vendor’s documentation and guidance in regards to setting up the switches for multicast traffic. 1.4 Multicast guidelines for PowerHA SystemMirror 7.1 In this section set of guidelines will be provided in regards to multicast setup in your environment. However note that these guidelines are generic in nature and the configuration of the switches would depend on your network environment and switch type and capabilities. 1.4.1 Test Multicasting in Network Do not attempt to create the cluster until multicast traffic flows without any interruptions between the nodes that will be part of the cluster. From PowerHA SystemMirror 7.1.1 Page 4 of 6 PowerHA SystemMirror 7.1 and Multicasting Setup Dated: 1st Nov 2011 release onwards, clustering will not continue if mping test fails. If there are problems with the multicast communication in your network environment, contact the network administrator and review the switches involved and the setup needed. Once the setup is complete, retest the multicast communication. One of the simplest method to test end to end multicast communication is to use the mping command available on AIX. In Fig 1, start the mping command in receive mode on one Host (Say Host A) and then use mping command to send packets from the other Host (Host B). If multiple hosts will be part of the cluster, test end to end mping communication from each host to the other. Command mping can be invoked with a particular multicast address or it will choose a default multicast address. Following illustrates an invocation of mping command for a multicast address 228.168.101.43: Host A: mping Packet Receiver Host A> mping -v -r -c 5 -a 228.168.101.43 Host B: mping Packet Sender Host B> mping -v -s -c 5 -a 228.168.101.43 Use the actual multicast address that will be used during clustering (later) as the address input to mping. Note that CAA creates a default multicast address if one is not specified during cluster creation. This default multicast address is formed by combining (ORing) 228.0.0.0 with lower 24 bits of the IP address of the Host. Example: If the Host’s IP address is 9.3.199.45, then the default multicast address will be chosen as 228.3.199.45 1.4.2 Troubleshooting guidelines If mping command fails to receive packets from Host to Host in the network environment, there could be some issue in the network path in regards to multicast packet flow. Follow some of the general guidelines below to troubleshoot the issue: 1. Review the switch vendor’s documentation for guidelines in regards to switch setup. Some of the known switch guideline links are included in the reference. 2. Disable IGMP snooping on the switches. Most switches will allow for disabling IGMP snooping. If your network environment allows, disable the IGMP snooping and allow all multicast traffic to flow without any problems across switches. 3. If your network requirements does not allow snooping to be disabled: Debug the problem by disabling the IGMP snooping and then adding network components one at a time for snooping 4. Debug if necessary by eliminating the cascaded switch configurations (having only one switch between the Hosts). Page 5 of 6 PowerHA SystemMirror 7.1 and Multicasting Setup Page 6 of 6 Dated: 1st Nov 2011